In an OFDMA communication system, a frame structure is illustrated in FIG. 1.
Referring to FIG. 1, a frame 100 has a length of 5 ms, and is divided into a downlink frame 110 and an uplink frame 120. A Tx/Rx Transition Gap (TTG) 115 is a gap between the downlink frame 110 and the uplink frame 120. A base station performs switching from a transmission mode to a reception mode, and terminals perform switching from the reception mode to the transmission mode during this gap. A Rx/Tx Transmission Gap (RTG) 125 is a gap between the uplink frame 120 and a downlink frame subsequently transmitted. A base station performs switching from the reception mode to the transmission mode, and terminals perform switching from the transmission mode to the reception mode during this gap.
In addition, the downlink frame 110 includes a plurality of subframes 130, and transmits downlink data and downlink control information. Similarly, the uplink frame 120 includes a plurality of subframes, and transmits uplink data and uplink control information.
Examples of the uplink control information include uplink fast feedback information, a hybrid Automatic Repeat Request (ARQ) feedback (or ACK) channel, bandwidth request indicator information for requesting a resource of an uplink, and the like.
The uplink fast feedback information may include various information such as a full Signal-to-Noise Ratio (SNR) or a Carrier-to-Interference Ratio (CIR), a Modulation and Coding Scheme (MCS) level favored by a terminal, Flexible Frequency Reuse (FFR) information, a beamforming index, and the like. When a data block received from a downlink by a terminal is not decodable, the hybrid ARQ feedback channel transmits 1-bit information in order to request retransmission. The bandwidth request indicator is used so that a plurality of terminals are allocated a specific signal sequence or transmit an arbitrary selected signal sequence by contention, and a base station may determine whether each terminal requests a bandwidth.
Since amounts of the uplink fast feedback information, uplink hybrid ARQ feedback information, and bandwidth information are not large but are very important information for an operation of a communication system, high reliability in transmission of them needs to be guaranteed. However, to prevent a resource waste, a frequency-time axis resource is not allocated much to a physical channel for transmitting them in general. Therefore, for reliable transmission, an efficient modulation/demodulation method is desired.
In the conventional OFDMA communication system, to transmit/receive the uplink fast feedback information, the hybrid ARQ feedback information, and the bandwidth request indicator information, a non-coherent modulation/demodulation method that does not use channel estimation is used. In addition, to secure high reliability of the fast feedback information, a frequency diversity gain is obtained by transmitting the information via a plurality of different frequency resources.
Generally, one frame 110 includes forty-eight OFDM symbols, and a ratio of downlink frame:uplink frame is 5:3. The downlink frame 110 includes thirty OFDM symbols, and the uplink frame 120 includes eighteen OFDM symbols. Here, one subframe 130 includes six OFDM symbols.
Depending upon the situation, the one frame 110 includes forty-two OFDM symbols, and a ratio of downlink frame:uplink frame is 27:15. The downlink frame 110 includes twenty-seven OFDM symbols, and the uplink frame 120 includes fifteen OFDM symbols. In an irregular uplink frame structure, one uplink subframe includes five OFDM symbols.
Therefore, to apply a signal sequence for uplink control information to the conventional uplink frame structure (that is, in the case where one uplink subframe includes 6 OFDM symbols) in an irregular uplink frame structure, a portion of the signal sequence needs to be punctured or repeated. Accordingly, orthogonality/correlation between codes may be distorted, and serious performance deterioration may occur. In addition, since a portion of the signal sequence needs to be repeated, a resource efficiency is lowered.